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Date:   Thu, 11 Apr 2019 13:32:06 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     David Hildenbrand <david@redhat.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Oscar Salvador <osalvador@suse.de>,
        Pavel Tatashin <pasha.tatashin@soleen.com>,
        Wei Yang <richard.weiyang@gmail.com>, Qian Cai <cai@lca.pw>,
        Arun KS <arunks@codeaurora.org>,
        Mathieu Malaterre <malat@debian.org>
Subject: Re: [PATCH] mm/memory_hotplug: Drop memory device reference after
 find_memory_block()
Message-ID: <20190411113206.GU10383@dhcp22.suse.cz>
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 <0bbe632f-cb85-4a98-0c79-ded11cf39081@redhat.com>
 <20190411105617.GS10383@dhcp22.suse.cz>
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On Thu 11-04-19 13:18:07, David Hildenbrand wrote:
> On 11.04.19 12:56, Michal Hocko wrote:
> > On Thu 11-04-19 11:11:05, David Hildenbrand wrote:
> >> On 11.04.19 10:41, Michal Hocko wrote:
> >>> On Wed 10-04-19 12:14:55, David Hildenbrand wrote:
> >>>> While current node handling is probably terribly broken for memory block
> >>>> devices that span several nodes (only possible when added during boot,
> >>>> and something like that should be blocked completely), properly put the
> >>>> device reference we obtained via find_memory_block() to get the nid.
> >>>
> >>> The changelog could see some improvements I believe. (Half) stating
> >>> broken status of multinode memblock is not really useful without a wider
> >>> context so I would simply remove it. More to the point, it would be much
> >>> better to actually describe the actual problem and the user visible
> >>> effect.
> >>>
> >>> "
> >>> d0dc12e86b31 ("mm/memory_hotplug: optimize memory hotplug") has started
> >>> using find_memory_block to get a nodeid for the beginnig of the onlined
> >>> pfn range. The commit has missed that the memblock contains a reference
> >>> counted object and a missing put_device will leak the kobject behind
> >>> which ADD THE USER VISIBLE EFFECT HERE.
> >>> "
> >>
> >> I don't think mentioning the commit a second time is really needed.
> >>
> >> "
> >> Right now we are using find_memory_block() to get the node id for the
> >> pfn range to online. We are missing to drop a reference to the memory
> >> block device. While the device still gets unregistered via
> >> device_unregister(), resulting in no user visible problem, the device is
> >> never released via device_release(), resulting in a memory leak. Fix
> >> that by properly using a put_device().
> >> "
> > 
> > OK, sounds good to me. I was not sure about all the sysfs machinery
> > and the kobj dependencies but if there are no sysfs files leaking and
> > crashing upon a later access then a leak of a small amount of memory
> > that is not user controlable then this is not super urgent.
> > 
> > Thanks!
> 
> I think it can be triggered by onlining/offlining memory in a loop. 

which is a privileged operation so the impact is limited.

> But as you said, only leaks of small amount of memory.

Yes, as long as there are no other side sysfs related effects.
-- 
Michal Hocko
SUSE Labs